The present invention relates to a proton conductor and a single ion conductor both used for devices in which cation or proton transfer occurs, such as secondary batteries, fuel cells, hydrogen sensors, bioelectrodes or the like, and a method of producing the same.
As a kind of solid electrode having an ion conducting mechanism, there is a single ion conducting high molecular weight film in which counter ion species are fixed in a high molecular weight compound, and only ion species, that is, only either cations or anions are transferred. As the single ion conducting high molecular weight film, for example, Nafion (trademark) developed by E. I. Du Pont in 1969 is well known. Actual researches on the mechanism or the like were actively pursued in 1980s, and synthesis of some ion conductors such as a polyethylene glycol derivative in which a carboxylic acid group (carboxylic group) or a sulfonic acid group (sulfonic group) is bonded to a side chain, and the mechanisms of the ion conductors have been studied. After that, as attention has been given to technologies relating to fuel cells again, because of a growing interest in environmental issues, researches on the ion conductors as electrolyte films for solid electrolyte fuel cells have been actively pursued, and in recent years, a large number of researches has been pursued.
Accordingly, researches on single ion conductors, specifically proton conductors are actively pursued at present. Proton conducting compounds are widely applicable, and are expected to be applied in various fields including not only the electrolyte films for solid electrolyte fuel cells but also diaphragms for hydrogen sensors, biomimetic hydrogen transfer films, materials for electrochromic displays, proton conducting films for chemical reactors, electrolyte films for proton transfer type secondary batteries and so on, therefore, a large number of researches including basic researches have been pursued.
However, the single ion conductor has a disadvantage, that is, low ionic conductivity. In a polyethylene glycol ion conducting macromolecule or the like, temperature dependence of a segment motion of the macromolecule has a large influence upon ionic conductivity, so the macromolecule has not surpassed liquid conductors in ionic conductivity yet. Moreover, the proton conductor is highly susceptible to water (proton carrier) existing in a film, so in a range higher than 100° C. which is the boiling point of water, the number of proton carriers decreases, thereby the proton conductivity pronouncedly declines. Therefore, a problem that a temperature range in which the proton conductor can be practically used is limited to 100° C. or less arises.
Accordingly, for example, an electrolyte film in which an inorganic substance having an oxygen functional group is introduced into a proton conducting compound so as to develop higher material strength and higher proton conductivity has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-155744). Further, a complex film in which a high molecular weight compound having a large number of sulfonic acid groups and a cation transfer type high molecular weight compound with low glass transfer temperature are mixed with each other so as to obtain higher ionic conductivity and a wider operating temperature range has been proposed (for example, refer to Japanese Patent No. 2962360).
However, in a proton conductor disclosed in Japanese Unexamined Patent Application Publication No. 2001-155744, film strength thereof can be improved, but proton conductivity thereof still has dependence on water, so during use of the proton conductor, water control is required. Therefore, the conventional problems cannot be completely overcome. Moreover, in a proton conductor disclosed in Japanese Patent No. 2962360, the high molecular weight compound is used as an ion carrier, so even in a state in which no water exists, proton conduction is developed, however, the proton conduction is controlled by a segment motion of a macromolecule, thereby, conductivity without water is low. Therefore, there is a problem that further contrivance to practically use the proton conductor is required.
In view of the foregoing, it is an object of the present invention to provide a proton conductor and a single ion conductor both having higher conductivity and a wider operating temperature range, and a method of producing the same.